The invention relates to a gas-blast circuit breaker. A circuit breaker of this type is known for instance from U.S. Pat. No. 3,331,935. The known circuit breaker has a movable contact member cooperating with a stationary contact member and having a first piston which, on breaking, is moved along the inner wall of a cylinder to thus compress insulating gas present in the cylinder. A second piston which, on breaking, is moved by a tensioned spring in the opposite direction towards the first piston additionally enhances the compression of the insulating gas present in the cylinder at the start of the breaking process, whereby the breaking capacity under large currents is improved. The spring providing the driving force for the second piston must therefore be designed to handle large forces. In addition, the second piston must be released by an expensive catch system at the start of the breaking process.